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Does it suck? #225671
12/19/10 07:38 PM
12/19/10 07:38 PM

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I read somewhere that "low pressure actually sucks you forward, you are not being pushed upwind... but being sucked"

is this true? how can it be?
feel free to get as technical as you wish....

-- Have You Seen This? --
Re: Does it suck? [Re: ] #225672
12/19/10 09:20 PM
12/19/10 09:20 PM
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“an island in the Pacifi...
hobie1616 Offline
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“an island in the Pacifi...
You're kidding. Right?


US Sail Level 2 Instructor
US Sail Level 3 Coach
Re: Does it suck? [Re: hobie1616] #225677
12/20/10 05:33 AM
12/20/10 05:33 AM
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North-West Europe
Wouter Offline
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It is true.

The suction forces on your sails are larger then the overpressure forces and therefore you more "sucked" then "pushed" forward.

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: Does it suck? [Re: Wouter] #225678
12/20/10 06:18 AM
12/20/10 06:18 AM
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Richmond, Va
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soccerguy83 Offline
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Same concept as an airplane wing air has to move faster over the curved leward side of your sail than the windward side causing a low pressure on the leward side.


Brian C.
H14
H16
Re: Does it suck? [Re: soccerguy83] #225680
12/20/10 06:58 AM
12/20/10 06:58 AM
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Sebring, Florida.
Timbo Offline
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From Wikipedia:

Real-world application

Condensation visible over the upper surface of a wing caused by a pressure drop due to air accelerationIn modern everyday life there are many observations that can be successfully explained by application of Bernoulli's principle, even though no real fluid is entirely inviscid [19] and a small viscosity often has a large effect on the flow.

Bernoulli's Principle can be used to calculate the lift force on an airfoil if you know the behavior of the fluid flow in the vicinity of the foil. For example, if the air flowing past the top surface of an aircraft wing is moving faster than the air flowing past the bottom surface then Bernoulli's principle implies that the pressure on the surfaces of the wing will be lower above than below. This pressure difference results in an upwards lift force.[nb 1][20] Whenever the distribution of speed past the top and bottom surfaces of a wing is known, the lift forces can be calculated (to a good approximation) using Bernoulli's equations[21] – established by Bernoulli over a century before the first man-made wings were used for the purpose of flight. Bernoulli's principle does not explain why the air flows faster past the top of the wing and slower past the underside. To understand why, it is helpful to understand circulation, the Kutta condition, and the Kutta–Joukowski theorem.


Blade F16
#777
Re: Does it suck? [Re: Timbo] #225681
12/20/10 07:04 AM
12/20/10 07:04 AM
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Timbo Offline
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So...in our world, ie. racing sailboats, if you are going upwind, you are being Sucked. If you are going downwind, you are being Blown.

Sucked or Blown, that is the question. And here's some light math for you to figure it out:

value of effort
The plots of air strike sail on both sides so:

On the windward side efforts are atmospheric pressure, wind pressure, and virtually no depression due to wind.
On the leeward side efforts are atmospheric pressure, a bit of depression and almost no wind pressure.
To simplify the manipulation of these forces, the forces are summed into a single force and that the entire surface of the profile (sailing) in a simple formula (valid for airplane wings like a rudder, a sail, an anti Plan -drift) [7]:

[8]

with

E = effort that can give up the wind (see Max Q);
C = coefficient Aerodynamic
According to the Bernoulli, the maximum stress of wind or density of kinetic energy maximum for the entire surface of the sail:



The full expression of the force is:



with

F = lift, expressed in Newton
ρ (rho) = density air (ρ varies with the temperature and the pressure) ;
S = typical surface, for sail, it is the sail area in m²
C = coefficient Aerodynamic. Aerodynamic coefficient is unit-less, it is the sum of two percentages: the percentage of recovered energy leeward side + percentage of the recovered energy into the wind. For this reason, the coefficient aerodynamics can be greater than 1. It depends on the angle of upwind sailing.
V = Speed is the speed of the wind relative to the sail (Apparent wind) in m / s.
The sail is deformed by the wind and takes a form named Airfoil. When the flow of air around the profile is Laminar [9], the factor against depression in the wind becomes crucial. This effect is then called lift. Studies and theory to draw a sail [10] that:

Depression on the upper (leeward side) represents two thirds of the lift,
The pressure on the lower surface (facing the wind) represents one third of the lift.
[edit] Lift effect on sail
The study effect of lift can compare cases with and without lift [11]. A typical example is a gaff sail. The sail is rectangular and is approximately vertical. The sail has an area of 10 sqm, with 2.5m of foot by 4m of leech. The apparent wind is 8.3 m / s (about 30 km / h). The boat is supposed to uniform velocity, no wave. It does not heel, does not pitch. The density of air is set at: ρ = 1.2kg / m3

[edit] turbulent flow or downwind
The boat is running downwind. The shape of the sail is approximated by a plane perpendicular to the apparent wind.

The depression effect on the sail is second order, and therefore negligible, it remains:

On the windward side, efforts are atmospheric pressure and wind pressure
On the leeward side, there remains only the atmospheric pressure
Efforts to atmospheric pressure cancel out. There remains only pressure generated by the wind.

Roughly speaking, shock of parcel on the sail forward all their energy from wind in 90% of the surface of the sail. This means that the Cz or aerodynamic lift coefficient is equal to 0.9.



[edit] laminar flow
The boat is Close hauled. The wind has an angle of about 15 degrees with chord of the sail.

Because the setting of the sail at 15 ° relative to the apparent wind, the camber of the sail creates a lift. In other words, the effect of depression on the leeward side is not neglected. As air pressure forces cancel out, efforts remain are:

On the windward side, wind pressure,
On the leeward side, wind depression.
The only unknown is the drag coefficient to be estimated. Curve takes a good adjustment of sail is close to upper shape NACA 0012[12][13]. A sail less well adjusted or older technology (old rig), will be more hollow, more camber. The coefficient of aerodynamic lift will be higher but the sail will be less efficient (lower finesse). The profiles would be more suitable profiles as NACA 0015, NACA 0018 [14].

For a given profile, there are tables which giving the lift coefficient of the profile. The lift coefficient (Cz) depends on several variables:

Incidence (angle: apparent wind / Profile)
The lift hill of the sail, which depends on its extension,
The surface roughness and Reynolds number, which affect the flow of fluid (laminar, turbulent).
The coefficient is determined for a fluid stable and uniform, and a profile of infinite extension.

The Reynolds number is:

with

U - fluid velocity or apparent wind [m / s]
L- characteristic length or foot of the sail [m]
ν - kinematic viscosity fluid: ν = η / ρ [m / s]
ρ - density air [kg / m³]
μ - dynamic viscosity air [Pa] or Poiseuille [pl]
so for this sail about Re = 106

Under an incidence of 15 ° and a Reynolds number to one million, reached a NACA0012 Profile Cz 1.5 instead of 0.9 for 90 ° incidence.



The lift has increased by 50%. This also corresponds on sheet an increase of 50% effort for the same apparent wind.[15][16]



Blade F16
#777
Re: Does it suck? [Re: Timbo] #225682
12/20/10 07:17 AM
12/20/10 07:17 AM
Joined: Jan 2005
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Sebring, Florida.
Timbo Offline
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Aww crap, the formulas didn't copy/paste for some reason. Well, you get the general idea.


Blade F16
#777
Re: Does it suck? [Re: Timbo] #225683
12/20/10 07:21 AM
12/20/10 07:21 AM
Joined: Jan 2004
Posts: 1,884
Detroit, MI
mbounds Offline
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[Linked Image]
where

L is lift force,
ρ is air density
v is true airspeed,
A is planform area, and
CL is the lift coefficient at the desired angle of attack, Mach number, and Reynolds number

Also,
[Linked Image]
where:

L is the lift,
A is the wing surface area
p is the value of the pressure,
n is the normal unit vector pointing into the wing, and
k is the vertical unit vector, normal to the freestream direction.

Re: Does it suck? [Re: Timbo] #225684
12/20/10 07:22 AM
12/20/10 07:22 AM
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Tony_F18 Offline
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Should I be worried that airline pilots get their information from wikipedia? wink
(I'm flying next week).

Re: Does it suck? [Re: Tony_F18] #225686
12/20/10 07:34 AM
12/20/10 07:34 AM
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Sebring, Florida.
Timbo Offline
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Yes, be very affraid, and drive to your next destination.

We don't build them, we just fly them. Pull back, the houses get smaller, push foreward, the houses get bigger.

That's about all there is to it. Just ask those Quantas boys on that A380 with the exploding engine...


Blade F16
#777
Re: Does it suck? [Re: Timbo] #225687
12/20/10 07:41 AM
12/20/10 07:41 AM
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pgp Offline
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rofl! Clearly you need more time on the water.


Pete Pollard
Blade 702

'When you have a lot of things to do, it's best to get your nap out of the way first.

Re: Does it suck? [Re: pgp] #225688
12/20/10 07:47 AM
12/20/10 07:47 AM
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Posts: 119
Bernhards Bay, N.Y.
popeyez7 Offline
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~~ I don't care if I'am gettin "BLOWN" or "SUCKED" on my boat...... If it's "WET, It's "FUN" ~~~~ ;)~


~~Don't fear the ''SPEED''~~ Fear the''ADDICTION''
~17,18 Hobies,
~Jet boat~Speedster 150
~2 Kayaks~
~~~~VIETNAM VET. 69-71~~~~
Re: Does it suck? [Re: Timbo] #225689
12/20/10 07:47 AM
12/20/10 07:47 AM
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Tony_F18 Offline
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Originally Posted by Timbo
Yes, be very affraid, and drive to your next destination.

LOL, yes a transatlantic tunnel! Wouldnt that be awesome! laugh

Re: Does it suck? [Re: Tony_F18] #225690
12/20/10 07:53 AM
12/20/10 07:53 AM
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Sebring, Florida.
Timbo Offline
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You might want to ask the people who got stuck in the Chunnel first. And that's only a short tunnel (26mi?) under the English Channel.

I was sitting in a pub in Brighton the day the Chunnel opened many years ago, watching all the hoopla on the TV over the bar. There was a reporter asking the Britts on site what they thought about now being connected by road directly to France.

One guy said, "Well I don't think it's a good idea." The reporter asked him, "Why, don't you like France?" He said, "I LOVE France, it's the French I hate!"

Classic!


Blade F16
#777
Re: Does it suck? [Re: Timbo] #225692
12/20/10 08:06 AM
12/20/10 08:06 AM
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Posts: 160
North Carolina
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abbman Offline
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I think the term "suck" is misleading. How can an air molecule pull on anything? It's like the old egg in a bottle experiment. The egg is pushed into the bottle, not sucked or pulled in.


James
1983 Hobie 16'
Re: Does it suck? [Re: abbman] #225693
12/20/10 08:13 AM
12/20/10 08:13 AM
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Sebring, Florida.
Timbo Offline
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Define Suction.

Using your logic, the term should not exist.

In training they told us a jet engine works like this:

Suck, Squeeze, Bang, Blow. I think a recip works the same way, but with more moving parts...


Blade F16
#777
Re: Does it suck? [Re: abbman] #225694
12/20/10 08:23 AM
12/20/10 08:23 AM
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Naples, FL
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Tim, I take issue with the "no substance is invicid". Look up Bose-Einstein Condensate. This material has ZERO viscosity

Even better, scientists have actually used this super-cold substance to slow the speed of light to roughly the speed of your average bicycle (without being diffracted). Now you can actually say you're moving "faster than light" (under certain conditions, of course)


Jay

Re: Does it suck? [Re: waterbug_wpb] #225695
12/20/10 08:35 AM
12/20/10 08:35 AM
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pgp Offline
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So I should rub it on my sails?


Pete Pollard
Blade 702

'When you have a lot of things to do, it's best to get your nap out of the way first.

Re: Does it suck? [Re: pgp] #225696
12/20/10 08:46 AM
12/20/10 08:46 AM
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Naples, FL
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interesting thought. maybe wear gloves smile

It would be hard to time your finish, however, with all that quantum flux going on... But your finish time could be sooner than your start time... How do you handicap that?


Jay

Re: Does it suck? [Re: waterbug_wpb] #225698
12/20/10 08:59 AM
12/20/10 08:59 AM
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pgp Offline
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laugh Which side?


Pete Pollard
Blade 702

'When you have a lot of things to do, it's best to get your nap out of the way first.

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